1. Field of the Invention
The present invention relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same, and more particularly, to a non-aqueous electrolyte for a lithium secondary battery that prevents a battery from expanding when being stored at a high temperature, while maintaining the electrochemical properties and improving the safety of the battery.
2. Description of the Related Art
Due to recent trends toward more compact and lighter portable electronic equipment, there has been a growing need to develop a high performance and large capacity battery to power the portable electronic equipment. In particular, there has been extensive research to provide lithium secondary batteries with good safety characteristics and improved electrochemical properties. Lithium secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials. The positive active materials include lithium metal oxide, and the negative active materials include lithium metals, lithium-containing alloys, or materials that lithium ions such as crystalline or amorphous carbons, or carbon-containing composites.
A cross-sectional view of a general non-aqueous Li-ion cell is shown in FIG. 1. The Li-ion cell 1 is fabricated by inserting an electrode assembly 8 including a positive electrode 2, a negative electrode 4, and a separator 6 between the positive and negative electrodes into a battery case 10. An electrolyte 26 is injected into the battery case 10 and impregnated into the separator 6. The upper part of the case 10 is sealed with a cap plate 12 and a sealing gasket 14. The cap plate 12 has a safety vent 16 to release pressure. A positive electrode tab 18 and a negative electrode tab 20 are attached to the positive electrode 2 and negative electrode 4, respectively. Insulators 22 and 24 are installed on the lower part and the side part of the electrode assembly 8 to prevent a short circuit occurrence in the battery.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7 V, which is higher than other alkali batteries, Ni-MH batteries, Ni—Cd batteries, and the like. An electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2 V is required to generate the high driving voltage needed. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like, is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thus resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Since lithium has a high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, and the like, thus forming a thin film on the surface of the negative electrode. The film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions, into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, so that an amount of lithium ions is maintained. That is, the carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79-104). Hence, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72(1998), 66-70). The gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, and the like, depending on the type of non-aqueous organic solvent and the negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery, and the passivation layer is slowly disintegrated by electrochemical energy and heat energy, which increases with the passage of time when the battery is stored at a high temperature after being charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously.
The above problems occur in a positive electrode. At initial charging, positive active material reacts with electrolyte to form a passivation layer on the positive electrode, and the passivation layer prevents decomposition of electrolyte, resulting in maintenance of stable charge-discharge. As in the negative electrode, the charge consumed during formation of the passivation layer on the positive electrode is irreversible. Thus, in a lithium ion battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, the passivation layer is slowly disintegrated by electrochemical energy and heat energy, which increase with the passage of time when the fully charged battery is stored at high temperatures after being charged, for example, if the battery is stored at 85° C. for four days after a 100% charge at 4.2 V. Accordingly, a side reaction in which an exposed surface of the positive electrode reacts with surrounding electrolyte occurs continuously to generate gases. The generated gases include CO, CO2, CH4, C2H6, and the like, from decomposition of a carbonate-based solvent.
The internal pressure of the battery increases with the generation of gases in both positive and negative electrodes. The increase in the internal pressure induces the deformation of prismatic and lithium polymer batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly (positive and negative electrodes, and separator) of the battery occur, thus deteriorating the performance and safety of the battery and causing difficulty in mounting the lithium secondary battery set into electronic equipment.
Further, disintegration of the passivation layer due to an increase of electric or thermal energy results in a continuous side reaction between positive and negative electrodes and the electrode. Gases generated from the side reaction increase internal pressure inside the battery and incur deformation of the battery to induce a short or a thermal runaway.
For solving the internal pressure problem, a method improves the safety of a secondary battery including a non-aqueous electrolyte by mounting a vent or a current breaker to eject internal electrolyte solution when the internal pressure is increased above a predetermined level. However, a problem with the method is that mis-operation may result from an increase in internal pressure itself.
Furthermore, a method in which the SEI-forming reaction is changed by injecting additives into an electrolyte to inhibit the increase in internal pressure is known. For example, Japanese Patent Laid-open No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Patent Laid-open No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved using 1 to 20% of an N-butyl amine based compound in an electrolyte. Japanese Patent Laid-open No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−3 M of calcium salt to the electrolyte. Japanese Patent Laid-open No. 94-333596 discloses a method in which storage characteristics of a battery are improved by adding an azo-based compound to inhibit the reaction between an electrolyte and a negative electrode of the battery. In addition, Japanese Patent Laid-open No. 95-320779 discloses a method in which CO2 is added to an electrolyte, and Japanese Patent Laid-open No. 95-320779 discloses a method in which sulfide-based compounds are added to an electrolyte to prevent the electrolyte from decomposing.
The methods as described above induce the formation of an appropriate film on a negative electrode surface such as an organic SEI film by adding a small amount of organic or inorganic materials to improve the storage characteristics and safety of a battery. However, there are various problems with the above methods: the added compound is decomposed or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in electrons; and gas is generated inside the battery, increasing internal pressure and resulting in significant deterioration of the storage, safety, cycle life, and capacity characteristics of the battery.